Thermal mismatch is also a problem for semiconductor chips which are mounted on a metal substrate by a diffusion solder technique. In these packages, the semiconductor chips are electrically connected to the metal substrate by the bond between the rear surface of the chip and the substrate. A diffusion solder bond has the advantage that a thin bond structure, for example around 10 μm, is provided. This improves thermal dissipation as well as enabling the size of the package to be reduced. However, as the bond structure is thin, it is less able to absorb the stress which occurs due to the mismatch in thermal expansion coefficients.
It is known from DE 101 24 141 A1 to position a buffer layer directly on the rear side of the semiconductor chip. However, the buffer layer also has to fulfill the function of producing an ohmic contact to the silicon chip. The contact function limits the number of metals which can be used since only a limited number fulfill this requirement. Since these materials are not optimised for providing a mechanical decoupling, the stress absorption which is possible is limited and the degree of mechanical decoupling between the semiconductor chip and the die pad is limited.